The present invention relates to an optical modulation element for use in a display unit.
In a known ferroelectric liquid crystal panel, a ferroelectric liquid crystal formed into a thin film is interposed between opposed electrodes. At this time, several limited states of the ferroelectric liquid crystal becomes stable as shown in FIGS. 10a to 10c. In FIGS. 10a to 10c, reference numeral 100 denotes a molecule of the ferroelectric liquid crystal, reference numeral 101 denotes a cone, reference numeral 102 denotes an upper substrate and reference numeral 103 denotes a lower substrate. In FIGS. 10a and 10b, orientations of the molecules 100 of the liquid crystal are substantially uniform and spontaneous polarization of the molecules is directed upwardly or downwardly along a normal relative to the upper and lower substrates 102 and 103. In FIG. 10c, the molecules 100 of the liquid crystal are twisted in the direction of the normal relative to the upper and lower substrates 102 and 103. A direction of twist of the molecules 100 of the liquid crystal, which is opposite to that of FIG. 10c, may exist. Stable states of the molecules 100 of the liquid crystal may become different from those of FIGS. 10a to 10c according to angle of inclination of the molecules 100 of the liquid crystal relative to the upper and lower substrates 102 and 103 due to the kind of the orientation film and according the bending of the liquid crystal layer but can be schematically illustrated by FIGS. 10a to 10c basically.
FIGS. 11a to 11c are top plan views of the liquid crystal of FIGS. 10a to 10c observed from above the upper substrate 102, respectively. In FIGS. 11a to 11c, reference numeral 110 denotes the direction of spontaneous polarization, reference numeral 111 denotes a polarizer, reference numeral 112 denotes an analyzer, reference numeral 113 denotes a molecule of the liquid crystal in the vicinity of the upper substrate 102 and reference numeral 114 denotes a molecule of the liquid crystal in the vicinity of the lower substrate 103. When a liquid crystal panel is interposed between the polarizer 111 and the analyzer 112 intersecting at right angles, FIG. 11a corresponds to a bright state and FIG. 11b corresponds to a dark state. Thus, by using these uniform states in which the molecules 113 and 114 coincide, in position, with each other, the liquid crystal panel can give bright and dark displays. In the state of FIG. 11c in which the molecules 113 and 114 deviate from each other, the liquid crystal panel gives a gray display.
The ferroelectric liquid crystal formed into a thin film has such stable states as described above and is unstable in other states. Therefore, if a voltage applied to the liquid crystal panel is increased, changes among these states occur suddenly at specific values of the voltage. Thus, relationship between the voltage applied to the liquid crystal panel and the quantity of light transmitted through the liquid crystal panel exhibits sharp threshold characteristics. If a voltage not exceeding this threshold voltage is applied to the liquid crystal panel, the liquid crystal is kept stable. Hence, in the liquid panel, a display of high contrast and large capacity can be obtained in a simple matrix arrangement of only the electrodes without the need for providing a non-linear element at each pixel as in a thin-film transistor.
However, since the ferroelectric liquid crystal can assume a stable state restrictively as shown in FIGS. 11a to 11c, it is extremely difficult to achieve a number of gradations. The state of gray display of FIG. 11c is stable only in a narrow voltage region in which the state of FIG. 11a changes to the state of FIG. 11b. Thus, it is difficult to obtain a uniform intermediate gradation due to uniformity of the liquid crystal panel, etc.
Therefore, a prior art ferroelectric liquid crystal panel usually employs a binary display basically so as to obtain gradation through a plurality of pixels and a plurality of scannings as proposed, for example, by T. Leroux, F. Baume et al. in 1988 INTERNATIONAL DISPLAY RESEARCH CONFERENCE, p111-113. In this method, each of a scanning electrode and a signal electrode, which constitute each pixel, is required to be provided with one drive circuit.
Meanwhile, several methods are proposed in which the display of gradation is performed without increasing the number of the drive circuits. For example, in a method, the liquid crystal response threshold is changed by providing a region in which thickness of a liquid crystal layer varies in one pixel as proposed by Y. Iwai et al. in the Digest of 13th Liquid Crystal Conference (1987), p138-139. Meanwhile, in another method, an intermediate voltage is applied to an subelectrode by dividing a range between a voltage and a ground potential through a resistance element. Especially, regarding the latter method, a concrete example is reported in Japanese Laid-Open Patent Publication No. 63-316024 in which the main electrode and the subelectrode connected to a power source are connected by a resistance of 500 .OMEGA. and the subelectrode and the ground electrode are connected by a resistance of 500 .OMEGA.. However, this method has drawbacks in that the power consumption is quite large and stability of liquid crystal on the subelectrode is low. This is partly because in many drive methods, a positive or negative potential is applied to the sides of the scanning electrode and the signal electrode substantially at all times and thus, a rather large amount of current flows through the resistances substantially at all times. Furthermore, this may be partly because the subelectrode is grounded by the low resistance, so that drop of stability of the liquid crystal on the subelectrode is greatly influenced by nonselected pulses, thereby resulting in an unstable memory state.